The Radeon Pro are like the Quadro cards - ISV drivers (industry-specific certifications) and ECC memory at the expense of a little clock speed usually.
Most of the Radeon Pro cards don't have ECC memory, only the high-end versions do, usually the top one and maybe the one below the top. Everything else (which is the majority of the Pro/Firepro line) doesn't.
The WX9100 that replaced it isn't much faster and goes for $1500.
WX9100 is not the replacement for the Pro Duo - those are different lines and are very different cards. One is a dual mid-range Frankenstein board, and the other is a proper top of the line last gen. WX9100 replaces Firepro W9100, and not the Pro Duo. And their performance is also very different.
This Pro Duo card is two Polaris chips (500 series "updates", if I recall correctly) on a single PCB, with 16 GB of regular GDDR5 memory per chip. It's not the kind of card that makes sense for most workstation users, its only advantage is a lot of ram for cheap, but as it's spread out between 2 GPUs - not many workloads will be able to benefit from it. I think most workstation users will be better off with a used Vega FE - they also work with pro drivers, they'll be faster in most scenarios, and they sell for about the same price. This Pro Duo is a good deal, but only for those that don't need powerful GPU and will actually use all those 32GB routinely - that's a very narrow user base. It does make WX7100 pretty much irrelevant at this price though. But then again, while Vega FE cards are still available on the used market for ~ $500 - they're probably the best deal for most workstations.